Opposition transport spokesman David Davis has warned that Victorians will not accept a "half-baked" rail link from the city to Tullamarine, as he implored the state government to work with the Commonwealth to build a dedicated fast-train service.

Mr Davis seized on revelations the state government is considering a budget airport rail link that would scrap a tunnel to the city and stymie fast rail to Geelong, saying it would not be what Victorians were promised.

The Age reported on Saturday that the mayor of Geelong and a leading rail expert were calling on the state government to ditch the cheaper option, warning it would make airport services slow and unreliable.

Mr Davis said the state government appeared to have "gone cold" on a market-led proposal from a private consortium – including Melbourne Airport, Southern Cross Station and super fund giant IFM Investors – which has offered $5 billion on top of the $10 billion state and federal governments have pledged.

"What is required for the state government to do is take the money and work with the federal government to produce a proper airport rail link," Mr Davis said.

"We need a fast service from Southern Cross to the airport. It can't be scrambled in with a slow suburban service, it can't be scrambled in with a crowded suburban service."